Magnetic Vacuum Ovens (MVO's) have become essential tools in the manufacture of devices such as read/write heads for rigid media storage devices. Annealing is usually required at multiple stages in the manufacturing cycle--the initial plated wafer; the sliced wafer (bar); and the final chip (slider) stages.
The initial practice of placing a permanent magnet into standard laboratory vacuum ovens has several drawbacks:
Field Limitation: At any reasonable air gap, permanent magnets are limited to about 5 KGauss (KG). PA1 Cycle Times: Due to the large mass of the permanent magnets, the desired 3-4 hour cycle times must often be extended to 12 hours or more. PA1 Temperature Limitation: Permanent magnets will lose their magnetism if heated beyond the Curie point. PA1 High fields in the range of 20 KG can not be achieved at an efficiency of much better than 70%. (In terms of power--proportional to the square of the current, so that power efficiency is reduced by approximately 0.7.sup.2 .apprxeq.50%.) PA1 Tapering of the poles forces an increase in coil "diameter" with a resultant increase in mean turn, electrical resistance, and power. PA1 Saturation of pole corners at high (20 KG) fields forces larger poles in order to maintain reasonable field uniformity. PA1 High magnetic efficiencies (93-97% at 20 KG) PA1 Good product in/out access on one end, and good vacuum pump-out, power and instrumentation access on the other. PA1 Exceptionally good field uniformity in the x z plane. (Magnetic fields created by the coils themselves keep flux lines straight.) See FIG. 8. PA1 High Cost PA1 High Power PA1 Stray Fields
It is not usually feasible to place a permanent magnet around a vacuum-oven chamber due to field limitations (approximately 2 KG) imposed by the larger air gaps and high cost of large permanent magnets.
A number of MVO systems have been built with a vacuum-oven chamber in the air gap of "H" (FIG. 1) or "C" (FIG. 2) type electromagnets.
These types of electromagnets provide good air gap access and are quite practical in the 10-12 KG field range. Beyond this, however, saturation problems begin in the pole base, and forcing the system to higher fields becomes impractical in terms of power requirements. See FIGS. 3 and 4.
Tapering of the poles helps greatly but does not solve all problems:
Field Uniformity is usually defined as angular deviation of magnetic flux lines from those flux lines across the air gap at the pole center. This requirement ranges from 0.5.degree. to 7.0.degree.. Whether circular or rectangular, the pole must always be larger than the required uniformity area. Pole "shimming" may be used to improve field uniformities for a given pole size, but the resultant removal of material from the pole center results in lower fields for a given magnet excitation (Ampere-turns). See FIG. 5.
Obviously, the magnet poles are the root of many problems. Doing away with them entirely has resulted in the development (.about.1955) of the "window frame" magnet. See FIGS. 6 and 7.
Advantages of the window frame magnets include:
Disadvantages include:
A two axis winding machine is required and 8 bends vs. 4 bends for a "normal" flat coil. Both winding and potting tools are complex and expensive, and the entire process is labor intensive. PA2 Although magnetically efficient, the mean turn is obviously much longer than a "normal" rectangular coil. Resistance and required power are, therefore, proportionally increased. PA2 High stray fields are a hallmark of all electromagnets depicted up to this point. The window-frame magnet with saddle coils is especially troublesome in this regard since the coils are unavoidably further back from the air gap surface. See FIG. 9.
Note that in the y z plane, the core width must be larger than the "good field" area due to flux line bowing as we approach the ends, just as with magnets with poles.
It is therefore an object of the present invention to provide a vacuum oven with the capability of applying a strong (20 KG+), uniform electromagnetic field to a workpiece.
It is a further object of the present invention to provide a magnetic vacuum oven with a central chamber accessible through a "safe door" through the magnet core.
It is a still further object of the present invention to provide a magnetic field with minimal stray fields which are deleterious to ancillary equipment, e.g. vacuum gauges, computer monitors, etc.
It is yet another object of the present invention to provide an electromagnet with no poles or with minimal poles. Short poles may be incorporated with lengths limited to the point where decreased magnetic efficiencies (power increase) begin to outweigh the benefit of provided room for coils (more copper or aluminum conductor.fwdarw.lower power requirement). If poles are to be implemented, they should have as large an area to length ratio as possible. The larger the area/length ratio of the poles, the better the performance of the magnet.
It is another object of the present invention to provide separate product and vacuum pumpout/instrumentation access means.